As for me, a squad should be built like this :
1 Heavy Assault
2 Light Assault
1 Medic
1 Engineer
1 Infiltrator, specialized in sniping
As for vehicle squads :
3 Flash
2 Prowler ( TR all the way
)
2 Lightning at the front, along with the 3 Flash
1 Sunderer, protected by 2 Prowlers